The Objective2 (O2) Headphone Amp DIY Project

You're welcome... and good luck :)

(and as to opamp indestructibility, the only things that will realistically kill an opamp are incorrect supply polarity and/or feeding supply voltage into any of the other pins when the main supply rails are missing. Even then you have to be very very determined to kill it off by ensuring lots of current is available and also that it has somewhere to go through the other pins... and once its supplies are present its all pretty much protected anyway)

:xfingers:


Actually they are trivially destroyable if one of the supply pins disconnects while there is signal present, as that takes the inputs outside of the abs-max range. Assuming dual rail setup of course.

Worst case scenario: dual rail supply, some part fails and current limits the supply rails, causing one to pull the other across ground to reverse its polarity - all opamps can fry. So always incorporate backwards diodes between V+ and ground and between ground and V- if the regulators used don't already have such protection built-in (or even if they do). PSUs for large mixing consoles have even more protection features to prevent anomolous power rail situations that could endanger lots of devices.


Another failure mode is trying to use an opamp as a comparator when its not rated for it(!)
 
Administrator
Joined 2007
Paid Member
Thanks Mark, that is really what I was saying, reverse polarity and being able to force current into or out of the inputs when a rail has gone AWOL.

I wouldn't have thought the O2 with its AC coupling around those chips and the circuit impedance involved could generate those kind of destructive currents.
 
If I cant get MC7812ACTG, MC7912ACTG are MC7812BTG/MC7912BTG good replacement, difference is in tolerance as far I can tell, ACTG is 2% and BTG is 4%.

Screenshot-1.png

Screenshot-2.png
 
Greetings all. About a year ago i posted in this thread trying to fix my o2. I had attempted to move the input jack from the front of the amp to a rear 3.5mm jack, and in the process lost one channel of audio. I tried some of the recommendations in the thread, and some other fixes on my own, with no luck. I'll likely be building another o2 here soon, and would still like to have a rear mounted input, 3.5mm if possible, but I'm ok with RCA too.


Is there anything that I should be aware of going into building one of these with that in mind? anything I should plan for or avoid? Does anyone have any recommendations for RCA or 3.5mm ports I should use if I want to do this? any help would be great. I just want to make sure the amp works this time, and get back to using my headphones with a decent amp.


I'd also be fine with doing one more round of troubleshooting on the amp I have now, just not sure what the issue could be. Where we left off was checking for issues with the left/right channels by linking r3 and r7 with a wire. did that just a bit ago again, and it neither cut out nor gave me mono sound in both headphones, fwiw.
 
Last edited:
Administrator
Joined 2007
Paid Member
We would have to start again with the process... 12 months ago is a long time to pick up on.

R3 and R7 though. Look at the circuit.

If you link those as I suggested then you are linking L and R channels directly at the input to the circuitry. The result has to be mono if you still can hear normal audio at the output. If both channels are not identical in output (whatever that output may be) after doing this then that would point to a problem further in to the circuitry... but that would be unrelated to any previous work you did around the sockets.
 

Attachments

  • Annotation 2020-02-24 074733.png
    Annotation 2020-02-24 074733.png
    327.1 KB · Views: 163
We would have to start again with the process... 12 months ago is a long time to pick up on.

R3 and R7 though. Look at the circuit.

If you link those as I suggested then you are linking L and R channels directly at the input to the circuitry. The result has to be mono if you still can hear normal audio at the output. If both channels are not identical in output (whatever that output may be) after doing this then that would point to a problem further in to the circuitry... but that would be unrelated to any previous work you did around the sockets.
I completely understand, thanks for being wiling nonetheless! I tried shorting with a wire on either side of the legs, with the input soldered to P1, rather than J2. I wasn't sure if the side would matter (for R3/R7) so I tried both sides to each other, from the portion of the legs that is above the PCB. Should I try shorting again from the bottom, where the legs are soldered through?


In talking about this with passing, someone suggested, as did you, that something might have died earlier in the chain. Any choice culprits that would be easy enough to test for there? It just seems odd to me that I had no issues before the change in jacks, then they appeared, but it seems completely plausible that something else may have died in the short procedure, especially given the issues. Is there anything else that you would recommend I check? Hopefully I'll have some time after work.
 
Administrator
Joined 2007
Paid Member
You have to be logical with fault finding :) If we try a quick check like linking the resistors and that doesn't do what we expect then we have to begin with basics and work from the output stage forwards towards the inputs.

Linking those resistors is the same as linking pins 3 and 5 on U1 (always keep looking at the diagram to see it makes it sense). So i would try that, link the pins and see what output you get. Whatever you hear in the headphones should be identical left vs right.

If it is not then we jump further in toward the output stage to see why not.

Try that first though. Link the pins on the IC. You can do that on the underside of the board where the socket is soldered. Double check you have the correct pins linked before powering it up again.
 
So, I went to test connecting legs 3 and 5 out on U1 with wire, and no dice. However, I did retest connecting R3 and R7 again, and I did in fact get a result! The audio stopped on both channels, which suggests a short, if I'm not mistaken. With that in mind, what should I be looking for?
 
Administrator
Joined 2007
Paid Member
It could be a short somewhere. With it switched off try a quick resistance check from pin 3 to ground and then pin 5 to ground. Does either pin read short circuit to ground?

With it on also check that the DC voltage on each pin of the chip (apart from pins 4 and 8 which are the supplies) is zero.
 
Administrator
Joined 2007
Paid Member
The voltages are good :checked:

Pins 3 and 5 resistance checks suggest a problem. 11 ohms is very low. Look at the circuit and try and follow this...

You are using J2 ground (that is OK to use) but only seeing 11 ohms to pins 3 and 5. So a couple of things to check are:

1/ That you still read 11 ohms to the relevant IC socket pins with the IC removed. That's an important check because I don't know how your meter responds to the presence of other in circuit parts like the chip that may appear in parallel to what we want to measure.

2/ We also need to be sure your meter is reading correctly and that you are interpreting the results correctly. So as a quick check your meter should read 1 ohm if you quickly measure those resistors at the output IC's.

3/ Assuming the 11 ohms is a real result you now need to check the resistance between ground and pins 1 and 3 of plug P1.

Keep looking at the circuit.

If you have 11 ohms at pin 3 and 5 you should see 11 ohms PLUS the value of the resistor R7 or R3 added to the result.

Fwiw, if you can't see any obvious problems anywhere then the most like areas are around those sockets.

A definitive test would be to lift the resistors R3 and R7 and to feed your audio directly into pins 3 and 5 thus eliminating all the sockets etc.
 
I foolishly didn't double check before soldering, and installed Q1 backwards. I must have damaged it with the extraction and re-soldering, because the voltage from ground to the striped end of D1 is < 2 volts. Voltage from ground to non-striped end of D5 is -12.1 volts. Am I right in thinking Q1 is toast and Q2 is ok?
Thanks-------Mark
 
Administrator
Joined 2007
Paid Member
Q1 and Q2 are only used as series switches and so there should be no consequential damage to other parts of the circuit from inserting them incorrectly.

There are really only a few possibilities here:

1/ The voltage on D1 is being pulled down by a near short somewhere after the diode (that includes all the circuitry of the 02 after Q1 as well)

2/ Check the input voltage to the 7812. It should be +15 volts or higher.

3/ Check if the 7812 is getting hot. If it is then that could point to a short on the output.
 
Input voltage to the 7812 is only 0.34 volts; it does not get warm. Voltage between BT1 terminals is only 1.5 v, between BT2 terminals it is 12.06 v. Voltage in to the 7912 is -12.04 v. One resistance measurement is off range in the power off-resistance check measurements: R5 is 233 K, not 100 k.
 
Administrator
Joined 2007
Paid Member
So things to check now are:

1/ Is there DC voltage on the stripe end of D3? It should be 15V or higher. (I'm thinking cracked print somewhere)

2/ Are you certain the mains adapter is an 'AC Output' type and not DC. This is vitally important... sorry have to ask :)

(If it is a DC type then only one half of the power supply will work depending on polarity).

3/ If it is a correct AC type then check for AC voltage on the non stripe end of D3. You should see around 12 volts AC.

And that's about it :) I can't think of anything else that would give no input to the positive regulator.
 
No worries--yes, I am using a wall wart with an output of 18 VAC, 1 amp.
Very interesting--my measured values are about 2X what you expected: 28.9 volts dc at the stripe end of D3 and 21.2 VAC at the non-stripe end of D3. Does all of this indicate that Q1 is bad? (The O2 was never run with Q1 inserted backwards. I noticed the mistake before turning it on, but fear I damaged it when de-soldering it.) Replacement arrived today.
 
Administrator
Joined 2007
Paid Member
The voltages I quoted are the absolute minimums the O2 needs to operate correctly. Your 18 VAC input will give approximately 18*1.4 which is around 25.5 volts DC.

If you are seeing almost 29 volts DC then you will find the actual AC input is going to be closer to 20 to 21 volts AC. All normal because the wall wart voltage will be higher at light loading.

Q1 always sees just the stable and regulated supply from the 7812 and so the actual AC input voltage to the power supply doesn't matter.

So if you have this 29 volts DC on D3 then that same voltage should appear on the input pin to the regulator and yet you mentioned only 0.34 V at that point. I would recheck that measurement and if the 29V isn't on the pin then there has to be a break.